Two approaches have been proposed to control breakage. Zwamborn and Van 

 Niekerk, (1981, 1982) surveyed the performance of dolos-armored breakwaters 

 worldwide and concluded that most structures that failed had been under- 

 designed or had experienced construction difficulties. They formulated lower 

 values for the stability coefficients to produce heavier armor units which 

 would be stable against any crack-causing movement such as rocking in place 

 under wave action. Their results are reflected in Table 7-8. Reinforcement 

 of units with steel bar and fibers (Magoon and Shimizer, 1971) has been tried 

 on several structures. Markle and Davidson (1984b) have surveyed the breakage 

 of reinforced and unreinforced armor units on Corps structures and have found 

 field tests to be inconclusive. No proven analytical method is known for 

 predicting what wave conditions will cause breakage or what type or amount of 

 reinforcement will prevent it. 



Projects using tetrapods, tribars, quadripods, and dolosse in the United 

 States are listed in Table 7-11. 



g. Design of Structure Cross-Section . A rubble structure is normally 

 composed of a bedding layer and a core of quarry-run stone covered by one or 

 more layers or larger stone and an exterior layer(s) of large quarrystone or 

 concrete armor units. Typical rubble-mound cross sections are shown in 

 Figures 7-116 and 7-117. Figure 7-116 illustrates cross-section features 

 typical of designs for breakwaters exposed to waves on one side (seaward) and 

 intended to allow minimal wave transmission to the other (leeward) side. 

 Breakwaters of this type are usually designed with crests elevated such that 

 overtopping occurs only in very severe storms with long return periods. 

 Figure 7-117 shows features common to designs where the breakwater may be 

 exposed to substantial wave action from both sides, such as the outer portions 

 of jetties, and where overtopping is allowed to be more frequent. Both 

 figures show both a more complex "idealized" cross section and a "recommended" 

 cross section. The idealized cross section provides more complete use of the 

 range of materials typically available from a quarry, but is more difficult to 

 construct. The recommended cross section takes into account some of the 

 practical problems involved in constructing submerged features. 



The right-hand column of the table in these figures gives the rock-size 

 gradation of each layer as a percent of the average layer rock size given in 

 the left-hand column. To prevent smaller rocks in an underlayer from being 

 pulled through an overlayer by wave action, the following criterion for filter 

 design (Sowers and Sowers, 1970) may be used to check the rock-size gradations 

 given in Figures 7-116 and 7-117. 



D (cover) 



5 D„^ (under) 



OJ 



where Dgc (under) is the diameter exceeded by the coarsest 15 percent of the 

 underlayer and Die (cover) is the diameter exceeded by the coarsest 85 

 percent of the layer immediately above the underlayer. 



7-225 



